1. Field of the Invention
The present invention relates to sorting or grading materials such as wood chips by size or thickness.
2. Description of the Related Art
In the processing of woodchips preparatory to introduction to a digester, it is preferred to reprocess chips which are thicker than a predetermined thickness and to discard those chip particles which have fibers shorter than a preset minimum length or which are in the form of flakes thinner than a preset thickness, because these are considered to be poor digesting material. For purposes of the present description, the chips to be reprocessed will be called xe2x80x9cover-thickxe2x80x9d and the undesired chip particles and flakes will be called xe2x80x9cfines.xe2x80x9d
Known devices and methods for separating acceptable chips from fines and over-thick chips include the use of vibrating or gyrating screens, disc screens, and oscillating bar screens. Other known devices for separating wood products are described in U.S. Pat. Nos. 5,109,988, 5,012,933 and 4,903,845. These patents describe the use of roll screens for the separation of woodchips for use in various industries. A roll screen comprises a plurality of rollers arranged parallel to one another in a screen bed. The rollers are provided with chip agitating protuberances. The protuberances may be knurls, grooves or ridges, and the rollers are rotated in the same direction so that the protuberances function to tumble and push the chips along the bed, from one roller to the next. The inter-roller dimensions, or gaps between rollers are sized to receive only the chips of proper thickness. As the rollers rotate, the acceptable chips and fines occupying the spaces between the rollers pass downwardly through the gaps into a hopper or into a discharge conveyor. The over-thick chips in the spaces between the rollers are nudged ahead by the oncoming chips and continue to be conveyed along the roller bed by the rollers for discharge from the forward end of the roller bed for reprocessing.
A second screen bed, having inter-roller dimensions selected to prevent acceptable chips from passing therebetween, is used to separate the fines from the acceptable chips.
Protuberances on the rollers may include knurls having various shapes such as pyramidal, conical, frusto-conical or frusto-pyramidal shapes. Ridges are preferably tapered and helical for the length of the rollers. Commonly, the formation of pyramidal or frusto-pyramidal knurls are formed by two helical sets of routed or machined V-grooves of opposite hand. Ridges are formed by single sets of helical grooves, either right- or left-handed. Commonly, when helical ridges are employed, the helical patterns of adjacent rollers in a roll screen alternate right-then left-handed, inasmuch as the helical groove of a rotating roller will tend to impart a lateral motion to the chips rolling across it. By alternating right-, then left-handed rollers, the chips will tend to move in a zigzag pattern as they progress down the bed of rollers.
There are several difficulties that are encountered in the employment of roller-type screens. For example, to increase the capacity of a roller screen, the length of the individual rollers can be increased. This allows material to be distributed across the entire length and permits the handling of larger volumes of material. However, as the length of the rollers increases, the possibility of contact between rollers also increases. As the rollers rotate and process material, the rollers may flex slightly. In the event that two adjacent rollers flex toward each other simultaneously, it is possible for the rollers to strike each other, resulting in damage to the knurled or grooved surfaces of the rollers. Naturally, rollers having a larger diameter will generally be more rigid than those having smaller diameters, and so, may be used to form wider beds without danger of strikes. However, the separation characteristics of rollers of different diameters vary, meaning that a screen made up of rollers of a larger diameter may be useful for sorting some kinds of chips or chip sizes, but not others. Thus, increasing the diameter of the rollers is not a universal solution.
Another difficulty encountered in roller screens is the need to maintain material evenly distributed across the width of the rollers, while preventing material from dropping off the ends of the rollers. Typically a sidewall on each side of the roll screen is provided for this purpose; however, small particles will still pass between the sidewall and the end of the roller. These particles will insinuate themselves into the drive trains and bearings of the rollers, necessitating periodic maintenance for the removal of foreign material.
The cost of the individual rollers is another consideration. As rollers are made longer and bigger, the cost of the rollers increases. Damaged rollers are more expensive to repair or replace, and the cost of maintaining a complete inventory of rollers for different applications can be prohibitive.
According to an embodiment of the invention, a roller is provided for sorting material according to physical dimensions. The roller includes a cylindrical sleeve having a first region with left hand spiral grooves and a second region with right hand spiral grooves. The sleeve may also comprise a smooth region between the first and second regions, the smooth region having an outside diameter equal to or greater than outside diameters of the first or second regions.
According to another embodiment, a shaft is positioned within, and coaxial to, the cylindrical sleeve previously described. The shaft may be sized to fit snugly within the cylindrical sleeve, in which case the shaft is coupled to the sleeve such that the sleeve rotates with the shaft, or an outer diameter of the shaft may be substantially less than an inner diameter of the cylindrical sleeve, in which case the shaft and the cylindrical sleeve are maintained in a coaxial relationship by an intermediate spacer positioned in a space between the outer surface of the shaft and the inner surface of the sleeve. The shaft, spacer and sleeve are coupled such that rotational energy is transferred from the shaft, through the spacer to the sleeve. The shaft and sleeve together comprise a screen roller.
According to an embodiment of the invention, a screen roller is provided. The roller is configured to rotate as one of a plurality of rollers in a roller screen. The roller includes left hand spiral grooves formed in a first region of an outer surface of the roller, the first region extending longitudinally from a first end of the roller toward the center of the roller, and right hand spiral grooves formed in a second region of an outer surface of the roller, the second region extending longitudinally from a second end of the roller toward the center of the roller.
According to an embodiment of the invention, a screen roller is provided. The roller is configured to rotate as one of a plurality of rollers in a roller screen. The roller includes features on the outer surface thereof configured to agitate and screen material moving across the roller screen. A bumper strip is provided in a central region of the roller. The bumper strip comprises a smooth section of roller having a diameter sufficient that, in the event the roller flexes and strikes a neighboring roller, the bumper strip will make contact first, preventing damage to the roller or the features thereon.
An embodiment of the invention includes a roller screen having a plurality of rollers as described in one of the embodiments above.
According to an embodiment of the invention, a method is provided for screening material such as wood chips.